LATE BLIGHT is a serious fungal disease that was responsible for the Great Potato Famine in Ireland in the 1850's. The disease was first found in Montana in 1995. It can be spread through diseased seed potatoes, in both private gardens and in commercial potato operations. Late blight has the potential to devastate commercial potato crops in Montana. Because new strains of the late blight fungus have recently been found that are resistant to very effective and commonly used fungicides, such as metylaxyl, late blight is a very real threat to potato seed producers in Montana. It is important for home gardeners to watch for the disease and to take potato plants with suspicious symptoms to their county Extension agent and be prepared to quickly destroy plants if the disease is confirmed.
The disease affects and is spread by crops and weeds in the family Solanaceae, including potato, tomato, pepper, and weeds such as the nightshades. Symptoms usually appear first on older leaves soon after flowering, following warm and wet or humid weather. Dark green, water-soaked areas at leaf tips spread inward and become dark brown and brittle in one or two days. On undersides of these leaves a cottony film may be visible around the edges of the lesions. Lesions obliterate the pattern of leaf veination. (Another form of blight, early blight, does not obliterate patterns of leaf veination.) Potatoes near the soil surface can develop lesions that are irregular, sunken; the lesions usually appear in and around the eyes. Affected tissue is granular and reddish in appearance an can penetrate up to one inch into the tuber. If you suspect late blight, it is very important to report it immediately to your county Extension agent, listed in the phone book under county government, or to the Montana Department of Agriculture.
To control late blight;
EARLY BLIGHT is another disease of potatoes. The first signs are circular or irregular dark spots that develop on older leaves. As these spots enlarge, they develop concentric rings, giving a target effect. There is usually a chlorotic, or yellowish zone around the spots that fades into the normal green of the leaves. Collar rot, with dark, girdling lesions, develops at the soil line, and tubers develop shallow, circular decayed areas about 1/3 inch in diameter, surrounded by puckered skin. The disease is caused by a fungus which overwinters in plant debris, soil, and in other members of the same plant family, such as peppers and tomatoes. To manage the disease, as soon as you spot symptoms, begin applying fungicides such as mancozeb or chlorthalonil. To help prevent early blight, plant resistant cultivars recommended by your county Extension agent. High soil fertility can help reduce severity of this disease.
BLACKLEG is a bacterial disease that can infect potatoes. Symptoms on the plant include foliage that gradually yellows and dies early, with leaves that turn brown on the margins and curl upward. At the base of the stem of affected plants, you might find mushy light brown to inky-black lesions, or rotten spots. Tubers may form on stems above the ground. Below ground, rot of the potato tubers usually begins at the stem end and causes a black, slimy rot.
The bacteria usually come from contaminated seed and are then spread by insects, rain splash, and equipment. Once a plant is infected control is difficult, so prevention is usually best. Sometimes treatment with fixed copper fungicides can be effective. Purchase only certified seed potatoes to avoid problems with blackleg, phytophthora and other problems. Avoid over-watering and mechanical damage to plants. Limit early nutrient supply to keep top growth within reason. These can be obtained from plant nurseries or mail order supply sources.
VERTICILLIUM WILT is a fourth potato disease you might encounter. The symptoms of this disease begin to appear late in the season, when older, lower leaves become yellow, curl and roll, and show tipburn, then die. The inside of the plant stem turns yellow or brown. This disease is caused by a fungus. Again, prevention is the best management tool, although applications of a sulphur fungicide every 7-10 days will help in control. To prevent the disease, plant resistant varieties, and do not plant in areas where other infected plants have grown.
PREVENTION OF POTATO DISEASES: To prevent many of the potato diseases, rotate crops in the potato family on a 4-year cycle (minimum) in your garden plot. Rotate garden crops, with a minimum 4 year rotation cycle, planting all crops in the same family as potato in a different part of the garden each year. For more information on rotating garden crops and on plant families, see the fact sheet in this series "Rotating Garden Crops to Prevent Disease". Clean up garden debris well in the fall and do not compost diseased plant material.
References:
Flint, M.L. 1990. Pests of the Garden and Small Farm. University of California.